The purpose of this research is to test a set of interrelated hypotheses about the psychosocial benefits of racial/ethnic diversity in urban middle schools. It is hypothesized that greater diversity can benefit students' mental health, intergroup attitudes, and school adaptation via three mediating mechanisms: (1) decreases in perceived vulnerability; (2) the formation and maintenance of cross-ethnic friendships; and (3) the development of complex social identities. These mediating mechanisms are examined in Aims 1-3. Aim 4 explores the moderating influences of individual student characteristics and school structural characteristics such as academic tracking that may limit the mixing opportunities of students. The hypotheses outlined in the four aims will be examined in a 3-year longitudinal study of approximately 7200 6th grade students as they enter one of 24 urban middle schools in southern or northern California. The large sample will be ethnically diverse, comprised of Latino, African American, Asian, Pacific Islander, White, and multi-ethnic students. The 24 middle schools that these students attend will be carefully selected to vary in racial/ethnic composition and level of diversity. Data on students' perceived vulnerability, cross-ethnic friendships, and social identities as well as the outcomes of mental health, intergroup attitudes, and school adaptation will be gathered in Fall and Spring of 6th grade and in Spring of 7th and 8th grade, for a total of four waves of data. Structural equation modeling and multilevel modeling will be used to track growth and change in these interrelated variables over time. By studying pathways to successful adaptation, the overarching goal of the research is to examine the conditions under which greater school ethnic diversity can buffer many of the normative challenges of early adolescence. Findings from the project will provide new insights into the role of school ethnic diversity as a psychosocial protective factor and into the meaning of ethnic diversity as a multi-faceted and dynamic construct. From a social policy perspective, the results can also contribute to the discourse on legal options for achieving greater racial/ethnic diversity in our nation's urban public schools.